The bandgap of III-nitride materials, including (Al, Ga, In)—N and their alloys, extends from the very narrow gap of InN (0.7 eV) to the very wide gap of AlN (6.2 eV), making them highly suitable for optoelectronic applications such as light emitting diodes (LEDs), laser diodes, optical modulators, and detectors over a wide spectral range extending from the near infrared to the deep ultraviolet. Visible light LEDs can be obtained using InGaN in the active layers, while ultraviolet (UV) LEDs require the larger bandgap of AlGaN.
Visible spectrum LEDs based on InGaN and AlInGaP systems have reached maturity and are now in mass production. However, the development of UV LEDs is still hampered by a number of difficulties involving basic material properties of AlGaN alloys, especially those with high Al content. Compared to LEDs in the visible spectral range with external quantum efficiency (EQE, the ratio of extracted photons to injected electron-hole pairs) of more than 50%, deep UV LEDs, such as those emitting below 300 nm, have an EQE of only up to 1%.
UV LEDs with emission wavelengths in the range of 230-350 nm are expected to find a wide range of applications, most of which are based on the interaction between UV radiation and biological material [Khan et al., 2008]. Typical applications include surface sterilization, water purification, medical devices and biochemistry, light sources for ultra-high density optical recording, white lighting, fluorescence analysis, sensing, and zero-emission automobiles. Although under extensive research for many years, UV LEDs, especially those emitting below 300 nm, remain extremely inefficient when compared to their blue and green counterparts. For example, Hirayama et al. recently reported 10.5 mW single-chip LED operation at 282 nm and peak EQE of 1.2% [Hirayama et al., 2009].
The growth of III-nitrides onto the c-plane sapphire is well-established. However, III-nitride material grown on c-plane sapphire suffers from the presence of polarization fields due to the polar nature of crystal bonds, which lead to energy band bending and reduction of recombination efficiency in quantum heterostructures due to physical separation of electron-hole wave functions, commonly known as the Quantum Confined Stark Effect (QCSE). Due to lattice mismatch, III-nitride materials grown on sapphire suffer from a high density of defects such as dislocations and inversion domains. A number of methods have been developed to obtain high quality single crystal material for device applications, including optimization of the nucleation process and choice of buffer layers to accommodate lattice mismatch. Alternative substrates, such as (001) Si, lithium aluminum oxide (LiAlO3) and silicon carbide (SiC) of various crystallographic planes also have been used for certain applications. However, native GaN and AlN substrates are still under development and remain prohibitively expensive.